Titanium-based or titanium alloy-based metal matrix composites (TMMC) are of particularly great interest in the following areas: the aerospace and automotive industries, medical implants and chemical-resistant applications due to their high specific strength, their high stiffness, low weight, and relatively high wear resistance. The titanium or titanium alloy matrix in these composites are reinforced by fibers or particles which have a substantially higher hardness and elastic modulus than the matrix alloy. Reinforcing components should be thoroughly and uniformly dispersed in the volume of the matrix alloy to achieve the maximum mechanical properties of the composite material. In addition, the strength of the composite material depends on the size of the reinforcing particles, strength of the bond between the hard particles and the matrix, and the porosity of sintered composite materials.
Despite more than twenty years of experience in industrial applications, conventional TMMC are far from perfection and used on a limited scale in industrial applications. They do not completely realize the strength benefits of the reinforced structure due to not optimal composition and technology, and especially, due to remaining interconnecting porosity of resulting composite materials.
For example, the method for manufacturing the Ti-6Al-4V/TiC composite disclosed in the U.S. Pat. No. 5,722,037 provides the density of the resulting material only about 93% of the theoretical value even after vacuum sintering for 4 hours at 1300° C. The method includes formation of reinforcing TiC particles in the titanium matrix by chemical reaction with hydrocarbon gas that is more effective in the porous matrix than in the dense one.
In the U.S. Pat. No. 4,731,115 granted to Abkowitz, et al., a TiC/titanium alloy composite cladding material and process for manufacturing the same are disclosed, in which blended components are compacted by cold isostatic pressing and sintered at 2200-2250° F. However, this method does not provide sufficient density of the material, and to improve the density, the invention further includes encasing the sintered pre-form and hot isostatic pressing (HIP) at 1650-2600° F. followed by finish forging, rolling, or extruding. This method is not cost-effective due to additional HIP step and encasing (canning) that should be removed from the final product by grinding or chemical milling. Moreover, the HIP process does not permit production of articles with close tolerances of their sizes. The presence of encasing testifies that the sintered composite material has interconnected porosity that results in the necessity to protect against oxidation during the hot deformation steps.
T. Kaba, et al. (U.S. Pat. No. 5,534,353) proposed compacting a powdered component blend by cold isostatic pressing, atomizing the product by melting and spraying, and finally, sintering the atomized powder by HIP at 1100° C. (2012° F.). The final product has improved bending strength at room temperature, but includes atomizing in a protective atmosphere, and it still has an interconnected porosity which requires additional encapsulating step for the HIP with a consequent increase in production costs.
All previous technologies of fabricating dense titanium matrix composites from matrix and reinforcing powders have considerable drawbacks that make them undesirable in terms of density, strength, and ductility of resulting products, sufficient protection from oxidation, cost, and production capacity. The interconnected porosity causes very rapid oxidation of the reactive titanium powder to a substantial depth, and capsules or cases (that are required for subsequent consolidation to near full density in known inventions) do not fully protect the sintered article from rapid oxidation, and also increase production costs. A significant difference in structural and mechanical properties between sintered material and the capsule produced from non-reactive wrought metal results in non-uniform deformation and stress concentration in the TMMC during the hot deformation. Cracks occur in various areas of the sintered material during the first cycles of hot deformation because of interconnected porosity and stress concentration. These cracks do not allow maintaining a reliable and reproducible manufacturing process through forging or hot rolling.
Therefore, it would be desirable to provide (a) a high-strength and fully-dense titanium matrix composites having discontinuous porosity after sintering, and (b) a cost-effective method for producing such composites using blended elemental powders or combination of pre-alloyed and elemental metal powder blends, as well. A new composition and method should improve the mechanical performance of resulting materials and further eliminate destructive porosity and oxidation during subsequent high-temperature processing that is required in order to achieve a near full density with acceptable mechanical properties.
This present invention achieves this goal by using complex carbides as additional reinforcing components in the Ti/TiC composite structure, and by providing a method through which the sintered structure has only the discontinuous porosity at the near full density, while at the same time, the composite material exhibits acceptable mechanical properties in the as-sintered conditions, and/or it is manufactured during foregoing hot deformation without any encasing, canning, or encapsulating if more complicated shapes with improved size control of the finished parts or improved properties are required.